The present invention is related generally to the field of directional drilling and, more particularly, to electronic roll indexing compensation in a directional drilling system and method.
Drilling systems generally use a drill string which extends from a drill rig, positioned at the surface of the ground, to an inground boring tool or downhole arrangement which is connected to a foremost end of the drill string. The specific configuration of the downhole arrangement may vary substantially, depending on the specific type of drilling operation that is being performed. Generally, however, directional drilling utilizes some form of downhole assembly which allows for steering the drill head in a controlled manner. In one configuration, the downhole arrangement includes a drill head having an asymmetric face. So long as the drill string is rotated continuously, a straight borehole is formed, assuming uniform soil conditions. In order to steer the drill head, the asymmetric face is oriented in a desired position while the drill string is extended, thereby causing the asymmetric face to produce a lateral force which deviates the borehole. In another configuration, the drill arrangement utilizes a “bent sub” connected to the foremost end of the drill string, which may also be referred to as a “bend sub.” The bent sub is generally attached on one side to the drill string and on the other side to a mud motor which rotates a drill bit which is powered using mud that is pumped down the drill string from the drill rig under considerable pressure. The bent sub provides a slight angular offset of the downhole arrangement with respect to the overall axis of the drill string for purposes of steering. That is, when the bent sub is oriented in a particular direction, using the drill string, for a period of time as the drill string is extended, the angular offset of the bent sub causes the borehole to be deviated in that particular direction. Formation of a straight borehole, using a bent sub, is provided using appropriate rotation of the drill string.
As is evident from the foregoing discussions, directional drilling requires an awareness of the orientation of the steering mechanism at the inground end of the drill string. In the past, various approaches have been used in order to provide this awareness. With respect to drilling arrangements which utilize an asymmetric drill head, it should be appreciated that a roll orientation signal can be transmitted from a transmitter that is located in the drill head itself, such that the transmitter co-rotates with the drill head. In such an arrangement, simple mechanical expedients may be used such as, for example, indexing tabs so as to index a zero roll position of the transmitter with the asymmetric face of the drill head. In other forms of drilling arrangements such as, for example, those using a bent sub, considerably more complex approaches have been necessary in the prior art to index the transmitter to the steering mechanism, as will be described immediately hereinafter.
It should be appreciated that a typical bent sub arrangement including a mud motor is generally incapable of carrying a transmitter within the mud motor itself. This result generally obtains since the mud motor is a relatively complex and long assembly having a central rotating drive shaft which rotates the drill bit. A mechanical indexing arrangement for a transmitter is therefore difficult to provide since the transmitter is generally located in the drill string behind the mud motor and the bent sub. Moreover, there will generally be a threaded connection between the drill string, that supports the transmitter, and the bent sub. This threaded connection produces an arbitrary roll orientation therebetween. Accordingly, mechanical indexing arrangements, in the presence of a bent sub or mechanically similar arrangement, tend to be quite complex in order to appropriately index the transmitter zero roll orientation to the steering direction of the bent sub.
The present invention provides an electronic roll indexing arrangement and method which resolves the foregoing difficulties and concerns while providing still further advantages.